In particular, the invention relates to a holding device commonly known as “lamp holder” or “lamp socket”, adapted to associate a lamp with the respective energised electric contacts and to ensure interchangeability. Lamp holders, generally made of an electrically insulating material, further allow assembling of same to bodies of lighting apparatus of different kinds constituting spotlights or floodlights, for example.
In more detail, the present invention advantageously applies, as a lamp holding device, to the lamps of new conception identified by the abbreviation PGJ5, which lamps have a rated voltage of 250 V and an impulse voltage of 1500 V. Such lamps, like the lamp Philips® CDM 20 W 250V with ballast ignition providing impulse discharges up to 1500 V, consist of a bulb made of glass and having a substantially oval shape, from which the light is emitted.
Extending from the opposite side of the bulb is an end portion from which two electric-connection elements project the shape of which is adapted to enable association with respective electric contacts. In particular, the connection elements extend opposite each other and transversely to the longitudinal extension of the lamp.
Generally, each connection element has a substantially parallelepiped conformation with a square cross-section, in which four faces are defined at least one of which is arranged to be associated with a respective electric contact of the lamp holding device.
It is known that lamp holding devices of the above mentioned type consist of a ceramic body of a substantially cylindrical conformation in which a bore for passage of the lamp is formed. In more detail, the bore has a substantially elongated shape in which two opposite ends are defined to enable passage of the electric contacts.
In this manner when the lamp is inserted into the bore, the lamp is rotated around its longitudinal axis in such a manner that its end portions are disposed in correspondence with two electric contacts (bayonet coupling).
Generally, the electric contacts consist of laminar portions made of a conductive material (metal) and are inserted into suitable spaces formed in the ceramic body on opposite sides of the passage bore for the lamp. Each electric contact is connected to a power cord by means of a suitable welding, and has a conveniently shaped flat surface to get in contact with a respective face of a connection element.
In this manner, when the lamp is inserted in the ceramic body, the lamp itself is rotated until respective faces of the two connection elements abut against the respective flat surfaces of the electric contacts.
Also known are lamp-i holding devices in which the electric contacts consist of a resilient flat spring.
In this case, when the lamp is rotated about its longitudinal axis the connection element abuts against the spring which is maintained in a compressed state.
Advantageously, by the resilient thrust of the flat spring, the electric contact is maintained steadily associated with the connection element to ensure a continuous electric connection.
Known lamp holding devices further have suitable seats such arranged as to enable housing of fastening screws to associate the device itself with any frame, such as a structure of lighting apparatuses like spotlights or floodlights, for example.
The screws are fitted in respective through holes formed in the ceramic body and disposed on opposite sides of the lamp. To facilitate mounting operations of the device, the screws are inserted from the bulb portion of the lamp so as to arrange the respective head towards the outside of the ceramic body.
The above described known art however has some important drawbacks.
First of all, it is to be pointed out that the above described lamp holding devices are not very reliable as regards the electric connection between the device contacts and the lamp connection elements. In fact, due to possible movements of the holding device, the lamp may slightly rotate causing separation of the connection elements from the electric contacts.
Also in the described solution in which the electric contacts consist of a flat spring, an unintentional slight movement of the lamp can cause separation of the connection elements from the electric contacts, which will bring about lack of distribution of electric energy.
Furthermore, a further drawback resides in that known devices enable insertion and energising also of lamps having operating features different from the provided ones.
In fact, due to the structure of the electric contacts, any connection element can get into engagement with the electric contacts irrespective of its shape. It is in fact to be noted that an electric connection is defined through coupling of a face of each connection element with a respective flat surface of the electric contact. Therefore insertion of a lamp with a different type of connection is possible, such as a lamp with a connection of the “G9” type. Insertion of a non suitable lamp into the holding device gives rise to important drawbacks, such as burst of the lamp if it is overpowered because the operating features of the lamp are different from the expected ones.
A further drawback relates to positioning of the seats for the hooking screws distributed in the ceramic body. In fact the ceramic body, generally made of steatite, is very fragile. This fragility is due not only to the material of which the body is made, such as said ceramic material, but also to the fact that the body itself has a very wide cavity and very thin walls in order to enable a bayonet coupling of the lamp.
Therefore, tightening of the screws could cause breaks and cracks in the ceramic body which will give rise to damage of the whole device.
In addition, due the presence of a crack in the ceramic body no electric insulation exists between the energised parts and the screw. Under this situation, the screw head being is disposed externally and in sight could be dangerously energised.